SiC Power Devices - Lessons Learned and Prospects After 10 Years
of Commercial Availability
Peter Friedrichs
Infineon AG, Schottky-Str. 10, 91058 Erlangen, Germany,
Peter.friedrichs@infineon.com
Keywords: SiC, power semiconductors, Diodes, Transistors
Abstract
The contribution will comment on the role of silicon
carbide based power semiconductor devices in industrial
electronics with a focus on high power densities and
improved efficiency. It will be sketched how the physical
properties of SiC can be favorably used in order to
minimize volumes of electronic components and/or
reduce losses in power electronic systems. Specific device
concepts for diodes and transistors with their pro’s and
cons will be discussed. A special attention will be given to
recent developments in order to further improve the
outstanding reliability of SiC components. After a short
outlook into the future if high voltage components a final
discussion about upcoming application scenarios for
modern SiC power devices will be given.
semiconductor manufacturing lines are still in operation for
150mm silicon wafers this step will open up a huge potential
for further reducing manufacturing costs of SiC devices.
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INTRODUCTION
The interest of the power electronics community in
modern SiC based power semiconductors is nearly 20 years
after the first promising news about this technology stronger
than ever. The driving force for the use of SiC in power
electronics is the potential benefit to realize low loss and
very fast unipolar diodes and switches with blocking
voltages from 600 V up to several kV. Efficient solutions
and high power densities are the most important benefits in
applications.
Among other wide band gap semiconductors which
theoretically offer comparable features like SiC, silicon
carbide has gained an outstanding status regarding base
material quality and technological maturity. Nevertheless,
one of the main hurdles for a fast market penetration was the
SiC crystal size, quality and cost. In 1993 the SiC wafer size
was just 1inch in diameter with more than 1000 so called
micropipes per cm² (simply holes through the entire wafer).
Thus, only very small chips with tiny current ratings could
be fabricated by special technology equipment. Now, in
2011, the situation has clearly improved: 100mm wafers
with a micropipe density well below 5/cm² are well
established (Fig. 1) at substantially reduced area specific
wafer costs. 150mm wafers were presented by Cree last year
[1] with an outlook for commercial availability in 2012as a
next step for reducing the gap to silicon. Since many power
Fig.1 Reduction of Micropipes from > 1000/cm² @1“ (1993) to < 5/cm²
@ 100mm (2007) commercially available
Furthermore, an essential condition for an increased power
density will be fulfilled by the availability of power
electronic components allowing higher switching
frequencies at high blocking voltages. But more important,
mainly with respect to the ongoing discussion about
greenhouse effects the aspect of energy saving by power
electronics becomes a focus point for power electronics and
here SiC is an ideal tool to offer efficient solutions by its
ability to enable low loss systems.
Regarding commercial products, fast SiC Schottky
diodes in the voltage range between 300 V and 600 V were
introduced on the market 2001 by Infineon (1200V in 2006)
for the first time and later by Cree and STM (in 2009) [2].
Because of the virtually zero reverse charge storage of
Schottky diodes these diodes are the nearly ideal partner for
e.g. the Silicon CoolMOS™ switch in order to fully exploit
the high frequency capability of this “pair” in the large
growing market of power factor correction in high end
power supplies. The higher costs of the new technology are
(well) compensated by the reduced size of passive
components [3]. This application was a first and important
step in order to establish SiC devices power electronics
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community. Today, the achievable power ratings are shifted
towards higher values and modules for motor drives e.g. are
used in power electronics [4]. Also for these applications,
the first use will be in combination with powerful silicon
IGBT’s.
Important key features for a successful use of SiC power
semiconductors in industry are cost and reliability
considerations. Thus, power devices made from Silicon
Carbide should not only be considered on the pure electrical
performance data, but on more aspects in order to capture all
commercialization issues. How such considerations can be
taken into account already while designing the device will be
sketched on the example of recently released generations of
modern Schottky Barrier diodes. For transistor devices a
discussion of pro and con’s of competing concepts (mainly
MOSFET and JFET) will be given. Finally the status of high
voltage (>3kV) components made by SiC will be given,
again connected to a critical view in the existing mission
profiles and application opportunities.
In the concluding part a few considerations will be made
regarding the short and mid term use of SiC power devices.
It will be shown how efficiency driven solutions in solar
power e.g. are preferred entry markets and first drivers for
this technology. Provided that life cycle approaches are
taken into account also additional applications can be
addressed which on a first glance do not justify higher cost
for better efficiency.
high reverse bias and thus, from local overstress and device
failure. Finally, with respect to the peak power stress
discussed above, the pn-parts in the structure will modulate
the conductivity in surge mode (when VF exceeds the VBI of
the pn-diodes) and thus, can limit the forward voltage drop
at high current which will end up with a considerable lower
peak power stress [6].
The most recent development of a third generation of
SiC SBD’s was strongly connected to the use of advanced
packaging technologies. On the one hand it is known that
SiC has a very good thermal conductivity and thus, the
resulting power losses should be easily removed, however,
thermal simulations revealed a strong contribution of the
commonly used backside solder process to the overall
thermal impedance, acting simply spoken like a wall for the
heat flow. New disruptive assembly flows are mandatory to
handle this. In the newest generation of Infineon diodes this
topic is addressed by avoiding solder layers at all [7]. This
technology does not only offer a reduction of the Rth by a
factor of 2, but in addition a further limitation of the internal
peak temperatures as shown in Fig. 2 what is again
beneficial for the long term stability of the device in its
package.
Conventional solder
RECENT GENERATIONS OF SCHOTTKY BARRIER DIODES
Driving forces for the current development of next
generation SiC Schottky Barrier diodes are mainly improved
cost performance ratios and the strategy to show an
enhanced reliability compared to competing silicon or even
GaN based technologies. Mainly in order to meet the second
goal it is important to address the today limiting features
regarding stability of the devices taking into account the
very high power densities arising from the high current
densities of the relatively small SiC chips. This is somehow
in contradiction to the demands on reliability since in
general it is well known that power densities (and mainly
their peak values) should be limited for a high reliability (the
resulting power density increases with the square of the
current density). For addressing this challenge design
improvements can be very beneficial. Examples for diodes
are modern diodes with an in-built surge current handling
capability by bipolar boosts [5]. In such diodes the active
area of the diodes is no longer a pure Schottky contact
(metal-n-type semiconductor barrier), but a grid of pnjunctions is introduced which will provide several added
values to such a device. Firstly, the metal n-type
semiconductor interface can be shielded from high electric
field in reverse mode if the design is adapted accordingly.
Secondly, stable avalanche conditions can be achieved at the
pn-junctions of the device which is an additional reliability
feature since it protects a Schottky Barrier diode from too
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New assebmly
10ms puls
Device area
1.95mm²
Red : 32A pulse
Blue : 42A pulse
Fig. 2: Peak temperatures in the devices during pulse current stress,
comparison of standard solder assembly and a new solderless
solution
The lower Rth allows for a higher current handling capability
of a given chip-size and thus, also an economical benefit can
be worked out by using this new technology. This new
packaging technology enables current densities up to
700A/cm² in discrete packaging solutions.
Due to the improved material quality also higher power
ratings can be addressed successfully today. Examples are
the 1200V/600A PrimePACK™ mixed silicon-SiC modules
(silicon IGBT and SiC freewheeling diode) introduced by
Infineon in 2008 [2] or the first commercial motor drive
inverter using SiC devices released by Siemens in 2006
already [4].
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SiC MOSFET
Si MOSFET
Vg>Vth
ID
The development of SiC-based switching devices is
mainly directed to unipolar switching devices with a clear
focus on 1200V blocking voltage. Further trends such as
bipolar junction transistor structures, thyristors or GTO’s are
under development in different laboratories world-wide
(e.g.[8-11]). The following analysis will be concentrated to
unipolar power transistors only.
Compared to the diodes, the commercial release of SiC
power switches is a much more complex issue. Besides
technological challenges, the competition with silicon
devices needs to be considered. For blocking voltages up to
800V, the silicon MOSFET dominates the market. In
addition, the price level of silicon devices is clearly below
that estimated for SiC. Due to the smaller chip size, SiC has
a smaller input and Miller capacitance, but the crucial
parameter for potential applications– the output capacitance
is not much different compared to the recent CoolMOS
devices [1].
With increasing blocking voltage, the competing Si
device is the IGBT since MOSFETs with an Ubr>1000V are
rare in silicon. Because the silicon IGBT is able to meet the
requirements of the circuit designer today, the chance for
SiC to replace the silicon switch quickly is rather low. The
potential of SiC devices compared to IGBT’s can be found
in hard switched applications with frequencies exceeding
about 20kHz or in applications where the threshold voltage
(VCEsat) of IGBT’s degrades the partial load efficiency like in
modern solar power inverters. However, in general, a much
higher price difference compared to the MOSFET
considerations made regarding the competition of SiC with
MOSFETs must be justified.
Unipolar SiC power switching devices under
development are predominantly MOSFET type devices and
Junction Field Effect Transistors (JFET). Cree recently
released first commercial power MOSFETs for 1200V [12].
In contrast to the diodes, these devices offer in addition to
the superior dynamics at least theoretically also advantages
regarding the static losses. Theoretically means in this
sentence that such lower static losses will require quite large
active areas and therefore, from a cost point of view, this
feature will be difficult to utilize effectively.
State of the art SiC MOSFET structures are designed with
ultra short channels (e.g. 0.5µm) and thus, very high channel
densities [13]. This approach helps to get low specific onresistances despite the poor channel characteristics. Even
after using nitrous gases for the passivation of interface traps
[14] and thereby improving the performance of SiC
MOSFETs considerably, increasing the channel mobility
remains the most severe challenge of the SiC MOSFET
technology. Fig. 3 illustrates the effect by comparing the
transfer and output characteristics of SiC MOSFETs with the
target performance of modern silicon power MOSFETs. The
poor interface quality in the channel region results in a
shallow sub threshold slope. This is also reflected in the
output characteristic where the on resistance changes
continuously with varying gate voltage.
Vg≈Vth
Vg<Vth
-
Si-MOSFET
SiC MOSFET
VDS
Fig. 3: Transfer characteristic (left) and forward conducting I-V behavior
(right, output characteristic) of SiC MOSFETs compared to silicon
power MOSFETs
In JFET based devices the channel region is located in the
bulk. JFETs are able to offer very low on-resistances as well
and they do not have susceptive interfaces involved in the
current flow path. The area specific on-resistance can be
much lower compared to unipolar silicon based devices, the
ratio is even more in favor of SiC if one considers higher
operating temperatures because the temperature coefficient
of the on-resistance of SiC FETs is smaller than that of
silicon devices (see fig. 4).
0,4
600V CoolMOS
NC-1200V JFET
600V LV-JFET
1200V LV-JFET
0,3
R on (O hm )
SIC SWITCHING DEVICES
0,2
0,1
0
0
50
100
150
200
250
300
Temperature (°C)
Fig. 4 :Increase of the on-resistance with temperature for different device
type like a silicon CoolMOS, a normally off SiC VJFET and two types of
SiCED’s lateral-vertical channel VJFETs
At the moment, the JFET structure is favored for being a
first commercial SiC switch by Infineon, the pioneer in
offering high performance SiC power semiconductors.
Several tests in applications confirmed the outstanding
performance [15, 16] of these VJFETs. The simplicity of the
structure itself which has only pn-junctions as functional
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elements ensures an extremely high ruggedness as proven,
for example, by an outstanding cosmic ray resistance [17]
and superior avalanche ruggedness [18].
A change of mind is required and it seems to be
mandatory to offer together with such a switch a driving
concept, preferable based on existing components with slight
adaptations. Such solutions are under investigation [19]. In
any case, the option to realize a normally off switch using a
normally on VJFET is well known as the cascode principle
[20] or the recently favored direct driven JFET approach.
This solution represents another generic combination of
silicon and SiC where both materials are used effectively
and with respect to their individual performance advantages
(Silicon with an advanced and well developed MOS
interface and SiC with ruggedness and high voltage
capabilities).
level between charge compensated silicon based solutions
and the SiC transistors. Entering this huge market would
have a dramatic impact on the overall success of WBG
power devices; however, the precondition will be to meet
the today’s severe cost targets for a given Ron of silicon
devices. Additional performance advantages like internal
body diode, lower capacitances and higher ruggedness
are soft facts which help to promote this technology,
however, they probably are not as important as the cost in
this market.
Finally for the higher blocking voltages, diodes as first
commercial components are expected, delivering power
ratings like today used in medium voltage inverters by
paralleling in modules as already pointed out for the
lower blocking voltages. Whether this potential can be
fully utilized will be a challenge for the creative people
working on the corresponding system architecture.
HIGH VOLTAGE DEVICES
ACKNOWLEDGEMENTS
High voltage (Ubr>3kV) applications seem to be even
more attractive for SiC since here the performance
advantages compared to the established silicon devices are
much higher. However, those applications are limited with
respect to the market size and therefore not as attractive for
semiconductor manufacturers like the huge 600V or 1200V
segment. In addition, a lot of the performance advantages
offered by high voltage SiC components like faster
switching or higher blocking voltages per chip cannot be
used today because of missing peripheral components. For
high currents e.g. even today’s silicon IGBT’s are artificially
slowed down in order to manage the high di/dt values. For
very high blocking voltages per chip today the appropriate
packaging technology is a severe challenge, stability of
silicon gel as well as partial discharge in ceramics need to be
addressed in order to utilize the full benefits of SiC
commercially. Progress is made today mainly for high
voltage diodes which again can act like their 1200/1700V
counterparts as an enabler for improved IGBT performance
in high power modules [21].
APPLICATION OUTLOOK AND SUMMARY
In summary, the door is open for SiC based wide band
gap devices to become important players in the field of
power electronics, especially for future applications
requiring a high power density and system efficiency.
Mainly with respect to the ongoing discussion about
greenhouse effects the aspect of energy saving by power
electronics becomes more and more important and here
WGB components are an ideal tool to offer efficient
solutions by its ability to offer low loss systems. Full SiC
solution (transistors) in modules probably will cover the
range interesting for solar industry with several tens of
kW. An interesting competition is expected at the 600V
304
The author would like to acknowledge the contribution
to this work offered by the staff of SiCED and the SiC
development group of Infineon Technologies as well as
the support from Siemens, division Automation and
Drives (now Siemens Industry, drive Technology and
Industrial Automation).
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